Basically, he just came up with something using FP that was more demo-able than somewhat hacky things people were already doing in other languages/environments. (For which, he deserves tons of credit.)
Bret victor's demos really have nothing to do with FP. The textual code demos in his learnable programming essay are all JavaScript and fairly imperative.
As were a lot of the things you could do years earlier in Smalltalk. FP with referential transparency makes it a whole lot easier and slicker. (Can also do it with data flow analysis.)
